1. Field of the Invention
The present invention relates to a composite conductor suitable for connecting conductors of a feeding passage to a superconducting system such as MRI (magnetic resonance imaging), linear motor, SMES (superconducting magnetic energy storage), and the like, and in particular, to a composite conductor composed of a superconductor and a metal conductor joined to each other to allow carrying a large current at a low resistance.
2. Description of the Related Art
In superconducting equipment such as a superconducting magnet, for convenience of laying out wirings, some parts between the metal conductors and between the metal conductor and the superconductor require to be joined to each other. For example, in some cases, a current lead electrode part (formed of a metal conductor) by which a current of several hundred of amperes to several thousand of amperes is fed to the superconducting system under a cryogenic condition from an external power source device under a room temperature, and a superconductive lead wire stretched from a magnet, are joined to each other via a connecting conductor, as shown in FIG. 5.
As a connecting conductor used in such joint part is used in general a high-purity copper, which is relatively well balanced considering electric resistance value, cost, processability, and so forth. In FIG. 5, “1” designates the current lead placed on a power-supply side, “2” designates the superconducting lead wire stretched from the magnet (system side), and “3” designates the connecting conductor connecting the current lead 1 and the superconducting lead wire 2. Plate-like metal electrodes 1a and 1b are provided on both sides of the current lead 1, and one end 3a of the connecting conductor 3 is pressure-contact-fixed to one end of the metal electrode 1a by a screw-type clamp 4 (pressure-contact method). Further, the other end 3b of the connecting conductor 3 is welded to an end portion 2a of the superconducting lead wire 2 stretched from the magnet by a solder 5 (solder method). Here, the pressure-contact method using the screw-type clamp 4 allows easy attachment and detachment, but has a disadvantage that a drift, which is an irregular current flowing in the joint surface in a deviated manner, is liable to be generated. Also, the problem in the solder method is that although the drift is suppressed to be relatively small, it is difficult to join the metal conductor and the superconductor so as to assure the uniformity of the contact resistance in an interface of the joint part, and it is also remained to be difficult to suppress the generation of the drift, further involving trouble of attaching and detaching (for example, refer to Patent document 1).
(Patent Document 1)
Japanese Patent Application Laid-open No. Hei 5-198433
In recent years, with advancement in development of superconductive application apparatus, the compact superconductive application apparatus having high-level performance of the joint part, with little generation of Joule heat, and easily attachable and detachable, is required.
However, the problem in joint using the conventional metal conductor is that a large contact resistance caused by drift is generated at joint parts between an electrode part of the current lead and a superconducting lead wire, and in order to decrease such a contact resistance, the joint area requires to be increased, resulting in making a large joint part. In addition, the pressure-contact method allowing relatively easy attachment and detachment causes a large drift, resulting in a large contact resistance.